The Philly mouse, derived from the Swiss-Webster strain, develops a cataract about 6 weeks after birth. Initial results have shown that in the lenses of these animals the epithelial cells fail to undergo complete differentiation. Biochemically, a 27 kD protein appeared to be missing in the Philly lens. This 27 kD protein is the betaB2- crystallin; in the normal lens it is a heat-stable protein. Investigation of the Philly mouse revealed that mRNA with approximately the same size as the normal betaB2 mRNA is present in the Philly lens. Furthermore, it was shown that a protein present in the Philly lens is immunologically related to the betaB2 protein in the normal lens. This protein shares the same amino terminal as the normal betaB2 but lacks part of the carboxyl half of the protein. The altered protein is slightly smaller and has a more acidic isoelectric point than the normal lens betaB2-crystallin. We cloned and sequenced cDNAs for normal and Philly mouse betaB2- crystallin. The normal mouse betaB2 cDNA is 725 base pairs in length with 618 base pairs of open reading frame. Deduced amino acid sequences suggest that the normal mouse betaB2 lacks a C-terminal phosphorylation site that is found in other mammals. The Philly mouse has a deletion of 12 nucleotides in the area encoding its fourth motif. By polymerase chain reaction, we have shown that the deletion exists at the genomic level in the Philly mouse. The properties of the protein encoded with this deletion appear to be consistent with the earlier protein findings and may be responsible for cataract formation.